Among the disorders, which the invention concerns, are those involving abnormal and excessive bleeding due to destruction of blood platelets (“platelets”).
These disorders include, but are not restricted to, post-transfusion purpura (“PTP”) and post-transfusion platelet refractoriness (“PTPR”), which are suffered by some persons who receive blood, platelets, leukocyte concentrates, or plasma from other persons by transfusion or the like.
The disorders also include one that is suffered by fetuses and newborns and is known as “neonatal alloimmune thrombocytopenia” (“NATP”). This disorder can cause death of fetuses and serious birth defects or death of newborns. NATP is estimated to affect about 1 in 1000 newborns. In NATP, fetal platelets, which enter the mothers blood stream, induce production in the mother of antibodies directed against fetal platelets. These maternal antibodies then pass with the mothers blood into the fetus and mediate destruction of platelets in the fetus.
A mother, whose fetus or newborn suffers from NATP, is at increased risk of suffering PTP or PTPR.
When platelets from a first human (a “donor”) are introduced into the blood system of a second human (a “recipient”) by transfusion, through the placenta (in the case of fetal blood entering the mother), or the like, the recipient may mount an immune response against the platelets from the donor. Such an immune response is referred to as an “alloimmune” response, because it involves antibodies reacting against antigens of a different individual of the same species. The alloimmune response to platelets is due to an immune response of the recipient against “alloantigens” (antigens of the same species as that mounting the immune response) on platelets from the donor. These alloantigens are found on membrane glycoproteins that occur in the cell membranes, which define the outer surfaces of platelets (“platelet membranes”). In this invention, the glycoprotein is anchored to the membrane in an atypical manner through an anchor consisting of glycosylphosphatidylinositol (GPI), which anchors an extracellular domain or segment of the glycoprotein exposed to the outside of the platelet. It is thought that alloantibodies, which are generated in an alloimmune response against platelet alloantigens, interact with the extracellular domains of the alloantigens.
The platelet alloantigens that a person has are determined by the person's genetics. A donor, because of his or her genetics, may have a platelet alloantigen, which a recipient, who receives blood, platelets, leukocytes or plasma from the donor, does not have, because of the recipient's genetics. In such a situation, the immune system of the recipient may recognize the donor's alloantigen as “non-self,” and raise an immune response against, the platelet alloantigen, which the donor has but the recipient does not.
Membrane glycoprotein alloantigens have been characterised for both human red blood cells and human platelets. It is noteworthy, however, that they also occur on other cell types, such as leukocytes and endothelial cells, where they may also occasion various disorders through alloimmune responses.
Recognised classes of red blood cell and platelet alloantigens have been described, over the past 30 years, based on observations of antibody reactions occurring when blood recipients have been exposed to blood from donors.
A recent review of human platelet alloantigen systems is provided by Ouwehand, W., and Navarrete, C., in Molecular Haematology, Provan, D. and Gribben, J. eds. Blackwell (1999).
Several biallelic platelet alloantigen “systems” have been characterised. In each of these systems, there are two alloantigens, each of which is provided by one of two alleles of the gene comprising the system. Because each gene occurs twice in the normal human genome, a person can be homozygous for one or the other of the two alloantigens, or heterozygous for the two alloantigens, comprising a biallelic system. The alloantigens described to date occur on glycoprotein molecules which may exist in various forms (transmembrane, GPI-linked and soluble, for example). In such a case, the alloantigens are found on each of the variant forms of the glycoprotein. For all of the biallelic platelet alloantigen systems that have been characterised at the level of protein and gene sequences, it has been found in all cases, except for one, that the difference between the two alleles is based on a single nucleotide polymorphism in the relevant gene.
One biallelic system of human platelet alloantigens is the Gova/Govb biallelic system associated with CD109, a membrane glycoprotein which occurs on platelets and various other cell types, including leukocytes and endothelial cells. Each Gov allele corresponds to one CD109 glycoprotein (Sutherland, D. R. et al, 1991; Smith et al., 1995; Berry, J. et al., 2000), consistent with the known tissue distribution of CD109. The frequencies for the Gov alleles are 0.4 for Gova and 0.6 for Govb in the Caucasian population. Thus, in this population, 40.7% are heterozygous for the Gov alleles, and will not mount an alloimmune response due to Gov incompatibility (not possessing the Gov alloantigen found on platelets received from another). In contrast, 19.8% of Caucasians are homozygous for the Gova allele and thus may mount an immune response due to Gov alloantigen incompatibility against platelets received from anyone in the 80.5% of the Caucasian population that is not homozygous for the Gova allele, while 39.8% are homozygous for the Govb allele and thus may mount an immune response due to Gov alloantigen incompatibility against platelets received from anyone in the 60.2% of the Caucasian population that is not homozygous for the Govb allele.
As indicated above, alloimmunization based on Gov incompatibility (the introduction into the blood stream of donor platelets bearing a Gov alloantigen not carried by the recipient) can result in bleeding disorders due to platelet destruction, including NATP, PTPR, and PTP. The location of the Gov antigens within the CD109 molecule, and the nature of the CD109 polymorphism which underlies the Gova/Govb alloantigen (both at the protein and at the gene level), have not heretofore been known.
Furthermore, it has not heretofore been possible to generate non-human antibody (polyclonal or monoclonal), as from a rat, mouse, goat, chicken, or the like, with specificity for the Gova alloantigen but not the Govb alloantigen (or vice-versa) sufficient for use in an immunoassay, for typing for Gov phenotype using platelets or CD109 molecules.
Previously developed technology, involving gene-specific amplification of platelet RNA-derived cDNA, followed by the determination of the nucleotide sequence of the amplified DNA, has been applied successfully to the elucidation of the molecular basis of other biallelic platelet alloantigen systems (Newman et al., J. Clin. Invest. 82, 739-744 (1988); Newman et al., J. Clin. Invest. 83, 1778-1781 (1989) (P1A or HPA-1 system); Lyman et al., Blood 75, 2343-2348 (1990) (Bak or HPA-3system); Kuijpers et al., J. Clin. Invest. 89, 381-384 (1992) (HPA-2 or Ko system); Wang et al., J. Clin. Invest. 90, 2038-2043 (1992) (Pen system). With one exception, it has been found in each case that a single amino acid difference at a single position differentiates the amino acid sequences of the two alleles, and that this difference arises from a single allele-specific nucleotide substitution in the coding region of the mRNA and gene. There remains a need to elucidate the molecular basis of the biallelic Gov platelet alloantigen system.